1. Field of the Invention
The present invention relates to an ink set, particularly to an ink set capable of forming images with excellent ozone fastness.
2. Description of Prior Art
In recent years inkjet recording methods have been receiving attention. An inkjet recording method is a printing method whereby ink compositions are jetted as droplets and adhered to a recording medium such as paper. The method has the advantage that, with relatively inexpensive apparatus, high-resolution and high-quality images can be printed at high speed. Ink-jet recording apparatuses employing this method have gained wide market acceptance for their excellent print quality, low cost, relatively quiet performance and excellent graphic-forming capability. Among such inkjet recording apparatuses, a thermal (Bubble Jet (registered trademark)) printer and a piezoelectric drop-on-demand printer, in particular, have been commercially successful and widely used as printers for personal computers in the office and home.
In recent years, a plurality of color ink compositions have been used in inkjet recording methods to form color images and obtain recorded matters. Generally, color images are formed with three colors of ink compositions, yellow, magenta, and cyan or, if desired, four colors including a black ink composition. There are cases where color images are formed with six colors of ink compositions including a light cyan ink composition and a light magenta ink composition, or seven colors by adding a dark yellow ink composition. An ink set is a combination of two or more ink compositions.
In addition to having good coloring properties, it is desirable that the respective ink compositions for forming color images form favorable neutral colors when two or more ink compositions are combined, and cause no discoloration or fading in color of the obtained recorded matters during storage.
In the field of “photo quality” printing using color inkjet printers, improvements have been constantly made to inkjet heads, ink compositions, recording methods and media, and accordingly, the “photo” level has been reached, with images as good in quality as “silver halide photos” being obtained. Moreover, as the ink compositions and the media have been improved, attempts have also been made to enhance the storage stability of the images of the recorded matters obtained by using color inkjet printers. Light fastness of the images, in particular, has been improved to a practical level (for example, see Japanese Patent Laid-Open Publications 2000-290559 and 2001-288392). However, an image does not deteriorate only due to the effect of light, but also due to oxidation caused by oxidizing gases in the environment (for example, ozone), which further escalates fading and discoloration of the images. Accordingly, in order to further enhance storage stability, enhancement of the ozone fastness as well as the light fastness of the image formed with ink compositions on the recorded matter is desired. For that purpose, various studies have been conducted to enhance the ozone fastness of inkjet recorded matters, especially the ozone fastness of colorants (coloring materials) used in ink compositions (for example, see International Publication WO02/060994 and Japanese Patent Laid-Open Publication 2002-371214).
When the ozone fastness of a certain ink composition is remarkably inferior to that of the other ink compositions, colors formed with that certain ink composition fade or change earlier than the other colors, and the color tones of the entire image become unbalanced. As a result, an observer notices the quality deterioration of the image earlier than he or she would notice the color fading of an image formed solely with a single ink composition. Accordingly, in addition to enhancing the ozone fastness of the respective ink compositions constituting an ink set, it is desirable to equalize as much as possible the ozone fastness levels among the respective ink compositions, that is, to have the ozone-caused deterioration rates (for example, color fading rates) preferably as similar as possible between images formed with each respective ink composition. In other words, it is preferable that the respective ink compositions constituting the ink set have excellent ozone fastness and, at the same time, there is little difference in ozone fastness between them.
When an ink set includes two types, dark and light, of ink compositions of different color densities for the same color, images can be formed with various color densities with no granularity. Ink sets including two types of ink compositions of different color densities are mainly used in photo printing, but in most cases with photo printing, ink compositions of low color densities are generally used so that the granularity of the images can be mitigated or eliminated. Even patterns with an optical density of approximately 1.0 used as samples in ozone fastness evaluations are formed with ink compositions of low color densities. Accordingly, it is important to improve the ozone fastness of low color density ink compositions in order to enhance the ozone fastness of photo images in recorded matters as well as the ozone fastness of the entire ink set. High color density ink compositions, on the other hand, are used for printing full-color images and graphic art patterns, so it is also necessary to improve the ozone fastness of these ink compositions.
A black ink composition is often included in an ink set because it has the important role of providing contrast in images. If a black ink composition is included in an ink set, it is necessary that the black ink composition have excellent ozone fastness, and also that the ozone-caused deterioration rate of the black ink composition be not remarkably different from that of the other ink compositions in the ink set.
As described above, it is required that the respective ink compositions constituting an ink set have good ozone fastness, that there is a good balance of ozone fastness between the ink compositions, and that some colors do not fade or change earlier than other colors when exposed to ozone. Meanwhile, because improving ozone fastness of the respective ink compositions in the ink set may sometimes causes bronzing, it becomes necessary to reduce any bronzing. Bronzing refers to a phenomenon in which, when high duty printing is conducted, printed parts look reddish and floating. This phenomenon tends to occur when solid printing (daubing printing at 100% duty) is conducted with an ink composition containing a metal phtalocyanine dye as a coloring material (colorant). Bronzing may upset the color balance of an entire image and degrade the image quality.